6.1.2 About XFS Write Barriers

A write barrier assures file system consistency on storage
hardware that supports flushing of in-memory data to the
underlying device. This ability is particularly important for
write operations to an XFS journal that is held on a device with
a volatile write-back cache.

By default, an XFS file system is mounted with a write barrier.
If you create an XFS file system on a LUN that has a
battery-backed, non-volatile cache, using a write barrier
degrades I/O performance by requiring data to be flushed more
often than necessary. In such cases, you can remove the write
barrier by mounting the file system with the -o
nobarrier option to the mount
command.